Two populations of red blood cells coexist in the circulation of patients with paroxysmal nocturnal hemoglobinuria (PNH), one normal and one that is abnormally sensitive to complement-mediated lysis. We have previously shown that these two populations are not maintained by two distinct stem cells populations in the bone marrow. Rather, it appears that any PNH progenitor cell may have the potential to generate both normal and abnormal red cells during clonal expansion. In our previous experiments, we studied the generation of abnormal cells during differentiation by using in vitro colony culture assays and an antibody to a complement regulatory protein, decay accelerating factor (DAF), which is missing on the complement-sensitive red cells and may account for the susceptibility of PNH cells to complement action. Using the antibody to DAF and flow cytometry, we also found that all cells in the circulation express DAF at varying amounts. Compared to normal individuals, lymphocytes, monocytes, granulocytes, and platelets from PNH patients had lower DAF expression, but it was not possible to detect two distinct populations as was seen with red cells. A small population of DAF- cells are seen in the blood, bone marrow, and cells in the in vitro colony assays in normal individuals. The presence of DAF- cells in normal individuals may provide a clue to the development of PNH: these infrequent cells may exist normally and their proliferation may be favored followed marrow insult, resulting in the development of PNH.